1. Field of the Invention
This invention relates to an air-fuel ratio feedback control system for an internal combustion engine, more particularly to an air-fuel ratio feedback control system adapted for use in a multiple cylinder internal combustion engine for absorbing variance in air-fuel ratio between cylinders and converging the air-fuel ratio in each cylinder on a desired value with high accuracy.
2. Description of the Prior Art
It is a common practice to install an air-fuel ratio sensor in the exhaust system of an internal combustion engine and feedback-control the value detected by the sensor for regulating the amount of fuel supplied to a desired value. A system of this type is taught by Japanese Laid-open Patent Publication No. Sho 59-101562, for example.
When a single air-fuel ratio sensor is installed at an exhaust gas confluence point of the exhaust system of a multiple cylinder internal combustion engine with four, six or more cylinders, however, the output of the sensor represents a mixture of the values at all of the cylinders. Since the air-fuel ratios at the individual cylinders cannot be detected with high accuracy, therefore, they cannot be precisely controlled. As a result, the air-fuel mixture becomes lean at some cylinders and rich at others, and the quality of the exhaust emissions is degraded. While this problem can be overcome by installing a separate sensor for each cylinder, this increases costs to an unacceptable level and also gives rise to a problem regarding durability. In light of these circumstances, the assignee earlier proposed designing a model describing the exhaust system behavior, inputting the output of a single air-fuel ratio sensor disposed at the exhaust system confluence point to the model, and constructing an observer for estimating the air-fuel ratios at the individual cylinders. (Japanese Patent Application No. Hei 3-359338; Japanese Laid-open Patent Publication No. Hei 5-180040 which was filed in the United States under the number of 07/997,769 and in EPO under the number of 92311841.8)
It was found, however, that when the estimated values obtained in this manner are to be used for absorbing variance in air-fuel ratio between cylinders and converging the air-fuel ratio in each cylinder on a desired value with high accuracy, a problem arises regarding how the feedback gain (correction term or correction coefficient) should be set. For overcoming this problem, there is proposed conducting air-fuel ratio control by setting separate feedback gains for the individual cylinders and for all of the cylinders (confluence point) based on the output of a single O.sub.2 sensor disposed at the exhaust system confluence point. (Japanese Laid-open Patent Publication No. Hei 3-149330)
Since this latter method does not use such a model as is describing the behavior of the exhaust system proposed earlier by the assignee, however, the accuracy of the air-fuel ratio control at the individual cylinders is insufficient. In addition, the O.sub.2 sensor used for detecting the air-fuel ratio is not a wide-range air-fuel ratio sensor, namely, does produce an inverted output only in the vicinity of the stoichiometric air-fuel ratio and does not produce a detection output proportional to the oxygen concentration of the exhaust gas. Moreover, as the air-fuel ratio detection speed is slow, the method is also unsatisfactory in this respect.
This invention was accomplished for eliminating the aforesaid drawbacks of the prior art and its object is to provide an air-fuel ratio feedback control system for an internal combustion engine wherein absorption of variance in air-fuel ratio between cylinders and high-accuracy convergence on a desired value(s) of the air-fuel ratios in the individual cylinders are achieved by setting optimum feedback gains for the control based on the exhaust system confluence point air-fuel ratio and for the control based on the air-fuel ratios of the individual cylinders.
Another object of the invention is to provide an air-fuel ratio feedback control system for an internal combustion engine wherein the air-fuel ratios of the individual cylinders are feedback controlled to a desired value(s) with high accuracy using a model describing the behavior of the exhaust system and an observer.
Still another object of the invention is to provide an air-fuel ratio feedback control system for an internal combustion engine wherein even higher control accuracy is achieved without use of a model by feedback controlling the air-fuel ratios of the individual cylinders to a desired value(s) based on detected values produced by air-fuel ratio sensors disposed in the exhaust system in a number equal to the number of cylinders.
For realizing these objects, the present invention provides a system for controlling an air-fuel ratio of an air-fuel mixture supplied to each cylinder of a multicylinder internal combustion engine, including, a first feedback loop for converging a first air-fuel ratio at a location at least either at or downstream of a confluence point of an exhaust system to a first desired air-fuel ratio, and a second feedback loop for converging a second current air-fuel ratio at each cylinder to a second desired air-fuel ratio. The improvement comprises said first feedback loop and said second feedback loop are connected in series.